Previous studies in animals and humans suggest that the immune response plays an important role in reactivation of latent herpes simplex virus (HSV), but the precise nature of this role remains uncertain. Immunosuppressed renal transplant patients and patients with acute leukemia taking chemotherapy are theoretically excellent groups to study in order to determine specific immune abnormalities associated with HSV reactivation. The major purpose of this pilot study is to collect preliminary data essential for planning that larger study. These data include the percentage of patients in the 2 study groups with HSV antibody, the rate and severity of HSV lesion formation and the rate of HSV shedding. The results of the pilot study will help determine the sample size, the length of time, the frequency of samples, and control group necessary for the larger study. This pilot study also has two other goals. First, to clarify the results of past studies by comparing the relative rates of HSV reactivation in kidney transplant and leukemic groups using standardized techniques. This is of interest since the immune deficiency differs somewhat in these two groups. Second, to determine the natural history of oral HSV infection in immunocompromised patients. This information is important to clinicians treating immunocompromised patients since HSV infection may be lifethreatening and is presently hard to distinguish clinically from oral mucosal lesions from other causes in immunosuppressed patients. This collaborative pilot study will utilize the facilities and personnel of the departments of Dental Medicine, Virology, Hematology-Oncology and Surgery of the Hospital of the University of Pennsylvania. The larger study will include the Department of Immunology. During a one-year period approximately 100 patients taking chemotherapy for acute leukemia and approximately 100 renal transplant patients taking immunosuppressive drugs will be tested for neutralizing antibody titers to HSV. Patients with HSV antibody will be evaluated daily for a one-month study period. Patients will be closely monitored for formation of facial, oral, skin and genital lesions and HSV cultures taken daily. All oral lesions will be cultured, classified, measured, and cytology smears obtained to detect cells with HSV changes. Oral surveillance HSV cultures will be obtained twice weekly to detect asymptomatic viral shedding. Data will be analyzed statistically to detect differences in oral HSV reactivation between the groups.